In recent years, there has been employed a phase difference detection method as a method for obtaining a tracking control signal from an optical disc on which information is recorded by concave and convex pits, as typified by a CD (Compact Disc) and a DVD (Digital Video Disc).
This method utilizes a phenomenon in which, when a light spot applied to an information recording surface of an optical disc passes over an information pit, an image (diffraction pattern) of the information pit on a photoreceptor changes due to a deviation of the light spot from the center of the information pit. To be specific, the photoreceptor is divided into plural elements in the direction along the track length of the image of the information pit, and an output signal level according to the amount of incident light to each photoreceptor element is observed. At this time, the change in the image of the information pit varies according to the direction and amount of deviation of the light spot from the center of the information pit, and the output signals from the respective photoreceptor elements are binarized at a predetermined level and then a difference in phases between the binarized signals is detected, thereby obtaining a tracking error signal that indicates the direction and amount of deviation of the light spot.
FIG. 5 is a block diagram illustrating the construction of an optical pickup for generating a tracking error signal by detecting a phase difference.
Hereinafter, a conventional method for generating a tracking control signal will be described with reference to FIG. 5.
FIG. 5 shows an example of an optical pickup employing a photoreceptor that is divided into four equal parts, and an astigmatism method for detecting a focus error signal. In FIG. 5, a light beam emitted from a light source 1, such as a semiconductor laser, is converted into a parallel light beam by a collimator lens 3. Thereafter, the parallel light beam travels through a half mirror 6, and is converged by an objective lens 4 to be applied, as a fine light spot, onto an information recording surface 51 of an information recording medium (optical disc) 5. A reflected light beam from the optical disc 5 travels through the objective lens 4, is turned to right in the figure by the half mirror 6, and travels through a convex lens 61 and a cylindrical lens 62 to be a converged light beam having two focuses which is a feature of the astigmatism method, and finally, reaches a photoreceptor 2.
When information is recorded by an information pit string having projections and depressions on the recording medium (optical disc 5), it is possible to obtain a tracking error signal which indicates a positional error between the light spot and the pit string (track) in the direction perpendicular to the track within the information recording surface.
Each of FIGS. 6(a)–(c), 7(a)–(c), and 8(a)–(c) shows the positional relationship between a light spot 12 and an information pit 13 when the light spot 12 passes over the pit 13 (6(a),7(a),8(a)), a change in an intensity distribution pattern (far field pattern) of the amount of reflected light on the photoreceptor 2 (6(b),7(b),8(b)), and a signal obtained from the photoreceptor 2 (6(c),7(c),8(c)).
When the light spot 12 passes over the information pit 13, the far field pattern of the amount of reflected light changes timewise. When the light spot 12 passes along the center of the information pit 13 (i.e., the center of the track) as shown in FIGS. 7(a)–(c), the far field pattern changes symmetrically. When the light spot 12 passes along the left side of the information pit 13 as shown in FIGS. 6(a)–(c), the far field pattern changes so as to rotate clockwise. When the light spot 12 passes along the right side of the information pit 13 as shown in FIGS. 8(a)–(c), the far field pattern changes so as to rotate counterclockwise. This rotating change of the far field pattern becomes sharper as the light spot 12 deviates more from the center of the information pin 13. The phase difference detection method utilizes the change of the far field pattern to detect a tracking error signal.
As shown in FIGS. 6(a) to 8(c), the photoreceptor 2 is divided into four elements 2a, 2b, 2c, and 2d, and a photoelectric current output according to the amount of incident light to each photoreceptor element is converted into a voltage signal by a current-to-voltage converter. By comparing the phases of sum signals each obtained from two photoreceptor elements that are diagonally placed, a positional error between the light spot 12 and the track 13 can be detected from the amount of advance or delay of the phase.
FIG. 9 is a block diagram illustrating an example of a signal processing circuit for detecting a phase difference, and FIGS. 10(a)–(h) illustrates signal waveforms outputted from the respective components of the signal processing circuit.
FIGS. 10(a)–(h) show a situation where the light spot 12 passes over the information pit (track) 13 from left to right with the passage of time, that is, the light spot 12 passes over the information pit 13 while changing from the state shown in FIGS. 6(a)–(c) to the state shown in FIGS. 8(a)–(c).
In FIG. 9, photoelectric current outputs, which have been detected by the photoreceptor elements 2a, 2b, 2c, and 2d into which the photoreceptor 2 is divided, are subjected to current-to-voltage conversion by current-to-voltage converters 7a, 7b, 7c, and 7d, respectively. Thereafter, the outputs from the current-to-voltage converters 7a and 7c are added by an adder 8a, and the outputs from the current-to-voltage converters 7b and 7d are added by an adder 8b. In this way, two signals obtained from the diagonally placed photoreceptor elements are added. The sum signals, each obtained by adding the signals from the diagonally placed photoreceptor elements, have waveforms shown by FIGS. 10(a) and 10(b), respectively, and these signals are transmitted through binarization circuits 9a and 9b to obtain signals shown in FIGS. 10(c) and 10(d), respectively. The above-mentioned tracking error signal can be obtained by detecting a phase difference between these signals at the rising or falling edges. In the circuit structure shown in FIG. 9, a phase difference at the falling edges is detected using D type flip-flops (D-FF) 101a and 101b. Thereafter, detected time difference pulses (e) and (f) are converted into a pulse width modulated signal (g) by a differential detector 102, and the signal (g) is filtered by a low-pass filter 11 to obtain an analog tracking error signal (h).
In each D-FF, a terminal marked with a circle and indicated by “T” is a clock input terminal, and a terminal marked with a circle and indicated by “R” is a reset input terminal. When the reset terminal “R” is at a logic “L” level, an output from a “Q” terminal is unconditionally at the “L” level, and when the reset terminal “R” is at a logic “H” level, a logic level equal to that given to an input terminal “D” is outputted to the “Q” terminal at the falling of the “T” (clock) terminal from “H” to “L”.
As shown in FIG. 10(h), when the light spot is placed in the center of a specific track, a tracking error signal obtained by using the signal processing circuit becomes zero level, and when the light spot is shifted to left or right from the center, the tracking error signal becomes an approximately linear signal having a polarity according to the shift direction. When the light spot is observed over plural tracks, the approximately linear signal appears for every track, and further, the signal becomes zero level when the light spot is placed between adjacent tracks, resulting in a sawtooth shape waveform in which the tracking error signal is repeated for every track, as shown in FIG. 11.
In order to perform tracking servo control by using the tracking error signals having the sawtooth shape waveform as shown in FIG. 11, a tracking servo control system should be constructed such that the objective lens 4 is driven by a means that is generally called a tracking actuator, according to the signs (plus/minus) of the tracking error signals.
In the above-described tracking error detection method, since the tracking error signals are detected using analog signal processing, optimization by redesign must be carried out to cope with speedup of the optical recording/reproduction apparatus and an increase in the recording density on the optical disc. For example, when the recording density on the optical disc is increased, a high-frequency component of a read signal obtained from the photoreceptor is undesirably attenuated, and a phase error signal cannot be correctly detected. FIG. 12 shows a solution of this problem. In FIG. 12, the sum signals, each obtained by adding the two signals outputted from the diagonally-placed photoreceptor elements by the adder 8a (8b), are subjected to high-band emphasis by waveform equalization filters 15a and 15b, and then binarized by binarization circuits 9a and 9b, respectively, to obtain phase error signals, whereby the degradation of the high-frequency component due to an increase in the recording density can be compensated. When the waveform equalization filters 15a and 15b are constituted by analog FIR (Finite Impulse Response) filters, all-pass filters are required for constituting delay parts of the FIR filters.
However, when the processing speed of the optical recording/reproduction apparatus is increased, since the channel rate of the read data varies, the required amount of delay varies significantly, and therefore, optimization of the delay circuit must be carried out. Further, since the required high-band emphasis characteristics also vary as the recording density varies, optimization according to an increase in the recording density is also needed. As described above, in the conventional tracking error detection method using analog signal processing, it is difficult to cope with speedup of the optical recording/reproduction apparatus or an increase in the recording density on the optical disc. Further, since the conventional tracking error detection apparatus includes many processing units for analog signal processing, it is not suited to be integrated with neighboring digital signal processing units.
In order to solve the above-described problems, there is proposed a tracking error detection circuit employing digital signal processing. FIG. 13 is a block diagram illustrating an example of a tracking error detection circuit employing digital signal processing.
With reference to FIG. 13, a reflected light beam, which is obtained by applying a light spot on a track of an information recording medium, is received by a photoreceptor 2 that is divided into four elements 2a, 2b, 2c, and 2d, and a photoelectric current output according to the amount of incident light onto each of the photoreceptor elements 2a, 2b, 2c, and 2d is obtained. The photoelectric current outputs so obtained are converted into voltage signals by current-to-voltage converters 7a, 7b, 7c, and 7d, respectively. Thereafter, the outputs from the current-to-voltage converters 7a and 7c are added by an adder 8a, and the outputs from the current-to-voltage converters 7b and 7d are added by an adder 8b. In this way, the two signals obtained from the diagonally-placed photoreceptor elements are added, thereby obtaining two sum signals for performing phase comparison. The two sum signals so obtained are digitized by analog-to-digital converter (ADC) 16a and 16b, respectively. Next, two sequences of sampling data obtained by the respective ADCs 16a and 16b are filtered by interpolation filters 17a and 17b to obtain interpolation data among the sampling data. For example, as an interpolation method, a method of obtaining data by Nyquist interpolation using a digital FIR filter may be employed. When using, as tap coefficients of this digital FIR filter, coefficients obtained by incorporating coefficients for performing data interpolation and coefficients having high-frequency emphasis characteristics, one digital FIR filter can have two functions of data interpolation and high-frequency emphasis. Next, zero cross points at the rising or falling edges of the interpolated two data sequences are detected by zero cross point detectors 18a and 18b, respectively. For example, as a method for detecting zero cross points, a method of detecting a change point of the sign (+→−, or −→+) in each interpolated data sequence may be employed. Next, a phase error detector 19 detects a phase difference from the distance between the corresponding zero cross points in the waveforms of the two data sequences, and outputs a phase error signal. The operation of the phase error detector 19 will be later described. Finally, the phase error signal so obtained is subjected to band restriction by a low-pass filter (LPF) 11, thereby obtaining a tracking error signal.
Hereinafter, the operation of the phase error detector 19 will be described with reference to FIGS. 14(a)–14(c).
FIGS. 14(a) and 14(b) show two data sequences for obtaining a phase difference, and FIG. 14(c) shows a phase error signal obtained by the phase error detector 19. In FIGS. 14(a) and 14(b), white circles (◯) indicate sampling data obtained by the ACDs 16a and 16b, white triangles (Δ) indicate interpolation data obtained from the sampling data sequence by the interpolation filters 17a and 17b, and black circles (●) and black triangles (▴) indicate zero cross points obtained from the sampling data sequence and the interpolation data sequence. The phase error signal shown in FIG. 14(c) is obtained from a specific single track and its vicinity, at the falling edges of the two data sequences. Further, the number of interpolation data is n=3.
When comparing the zero cross points shown in FIG. 14(a) and the zero cross points shown in FIG. 14(b), the amount of the phase difference between the two waveforms is in proportion to the distance between the corresponding zero cross points in the two waveforms. The direction of phase shift can be obtained by judging which one of the zero cross points of the two waveforms is the first to cross the zero level. The phase error signal shown in FIG. 14(c) is obtained from the amount of phase difference and the direction of phase shift which are thus obtained.
The obtained phase error signal is an approximately linear signal when attention is given to a specific single track and its vicinity. When it is observed over plural tracks, an approximately sawtooth-shape waveform in which the phase error signal is repeated for every track, is obtained as shown in FIG. 11. Finally, the phase error signal is subjected to band restriction by the LPF 11, resulting in a tracking error signal of a frequency band that is required for tracking servo control.
Since the above-described tracking error detection apparatus generates a tracking error signal by using digital signal processing, adjustment in accordance with an increase in the processing speed and an increase in the recording density is facilitated. Further, the signal processing after the ADC can easily be integrated with the neighboring digital signal processing units, resulting in a considerable reduction in the number of processing blocks which are required for analog signal processing.
In the tracking error detection apparatus employing digital signal processing, since the sampling data sequences are subjected to data interpolation to obtain the tracking error signal, the operation of the apparatus significantly varies depending on the frequency of the sampling clock. In the case of CLV (Constant Linear Velocity) reproduction, since reproduction is carried out so that the channel rate in the reproduced waveform becomes constant, stable tracking error signals can be obtained at both of the inner and outer circumferences of the disc by setting the sampling clock of the ADC in the tracking error detection apparatus to a fixed clock corresponding to the channel rate. However, in the case of CAV (Constant Angular Velocity) reproduction, the channel rate of the read waveform varies depending on the position of the pickup. For example, when the pickup is on the inner circumference of the disc, a tracking error signal can be obtained by setting the sampling clock to a fixed clock corresponding to the channel rate at this position. However, when the pickup moves toward the outer circumference of the disc, since the channel rate becomes higher than that at the inner circumference while the sampling clock is the fixed clock corresponding to the inner circumference, the interval of sampling performed on the read data is undesirably increased. In the tracking error detection apparatus employing digital signal processing, since the tracking error signal is obtained from the point number of interpolation data which are obtaining by interpolating the sampling data, the interval of the interpolation data is increased relatively to an increase in the sampling interval, whereby the amplitude of the obtained tracking error signal varies undesirably.
In order to solve the above-mentioned problems, there is proposed a method of using, as a sampling clock for CAV reproduction, a read clock that is generated in a read channel unit 20 which reads data from information pits on an information recording medium, as shown in FIG. 15.
FIG. 15 is a block diagram illustrating a read channel unit 20 to be used in a conventional digital system tracking error detection apparatus. In the read channel unit 20, an RF signal to be used for data reading (data reproduction), which is generated by amplifying and modulating a light beam that is reflected at an optical disc and detected by a photoreceptor, is digitized by an ADC 16c, and sampling data thereof are subjected to waveform equalization by a waveform equalization filter 25, and thereafter, data recorded on the optical disc are detected by a binarization circuit 26. In order to perform sampling at an appropriate timing in the ADC 16c, a PLL (Phase Locked Loop) circuit is employed.
In the PLL circuit shown in FIG. 15, a phase comparator 21 obtains a phase error signal from the sampling data of the ADC 16, a loop filter 22 filters the phase error signal, a digital-to-analog converter (DAC) 23 converts the filtered phase error signal into an analog voltage signal, and this analog voltage signal controls the oscillation frequency of a voltage controlled oscillator (VCO) 24. A clock output from the VCO 24 is inputted to the ADC 16 to operate the ADC 16, whereby the clock output serves as a read clock.
When the respective processing units for the above-described tracking error detection employing digital signal processing are operated by using the read clock generated in the read channel unit 20, stable tracking error signals can be generated at both of the inner and outer circumferences of the disc, by using the sampling clocks adaptive to the channel rates at the inner and outer circumferences of the disc.
In the above-described tracking error detection apparatus employing digital signal processing, although the read clock, which is generated in the read channel unit as an operation clock for the tracking error detection apparatus, is used to cope with the CAV reproduction, when the reproduction speed of the drive increases, the read clock also increases in proportion to the reproduction speed, resulting in an increase in power consumption of the tracking error detection apparatus.